This invention relates to a process for making high-hardness and high-toughness diamond composite materials, and in particular, diamond-metal, diamond-ceramic, and diamond-metal-ceramic (cermet) composite materials.
The prior art discloses the manufacture of composite materials by heating and compressing the composite powders. Mechanical pressures are generally used in the prior art. Various heating means are employed in the prior art, including ovens and the like. More recently, in the prior art, the heating is achieved by burning exothermic reaction mixtures such as thermite compositions, intermetallic reactions and the like. The exothermic powder is either mixed in the powder to be compressed, or fired as a separate layer adjacent to the composite material. Typical prior art methods are described below.
Certain kinds of ceramic, metal and cermet composite materials can be synthesized by utilizing an exothermic reaction between the elemental constituents of these materials without any external heating. This processing technique is termed "Self-Propagating High-Temperature Synthesis," and has been abbreviated as SHS, SHTS and SUS. SHS processing has been investigated in the Soviet Union since 1967, and over 200 phases have been produced by this technique. Exothermic reactions have been utilized for many years in the United States for a variety of heat requirements and pyrophoric applications. It is only recently that this processing technique has been explored as a method for synthesis and sintering of ceramic, metal and cermet composite materials.
In SHS processing, a strong exothermic reaction, ignited at one end of a compact of mixed precursory powders by heating (using electric spark, hot wire, ion beam, etc.), propagates spontaneously in the compact and passes through the whole sample. An example of this reaction is the synthesis of titanium diboride (TiB.sub.2) from titanium and boron powders. This reaction can be expressed as: EQU Ti+2B.fwdarw.TiB.sub.2 .DELTA.H =66.8 kcal/mol (at 298.degree. K.)
The adiabatic temperature resulting from this reaction is calculated to be 3190.degree. K. (assuming that all of the reaction heat contributes to increase the temperature of the reaction product), which corresponds to the melting point of TiB.sub.2. This particular reaction is self-sustaining. Self-sustaining reactions can occur only when a product phase is liquid or partially liquid at the reaction temperature. Therefore, not all exothermic reactions are self-sustaining.
Currently, fundamental research and applications of SHS processing are progressing. A. P. Hardt and P. V. Phung have proposed a simple, diffusion-limited, reaction model and evaluated the exothermic reaction rates. They have found that reaction rate depends on two factors: (1) a heat transfer which is sufficiently low to allow accumulation of heat in the reaction zone; and (2) system characteristics of particle size and fusion temperature which are sufficiently small to promote a high rate of mass transfer. Hardt and Phung have shown experimentally that reaction characteristics can be altered by using suitable additives which alter the thermal conductivity. Table 1 shows some influential factors of physical and chemical characteristics of precursory powders on SHS reaction and product.
TABLE 1 ______________________________________ Relation of SHS Characteristics to the Physical and Chemical Properties of the Starting Materials INFLUENCE OF INFLUENCE OF PRECURSOR PRECURSOR PHYSICAL CHAR- CHEMICAL CHAR- ACTERISTICS ACTERISTICS ______________________________________ REACTION Adiabatic Reaction Rate CHARACTER- Temperature Reaction Mechanism ISTICS Conductive Heat Loss Volatile Evolution Reaction Initiation PRODUCT Densities Secondary Phases CHARACTER- Microstructure Impurity Content ISTICS Unreacted Material Porosity ______________________________________
In the area of application research, J. D. Walton, Jr. and N. E. Poulos have applied the thermite reaction to production of high-temperature resistance cermets (1959). Self-bonding zirconium disilicide-aluminum oxide cermets (ZrSi.sub.2 -Al.sub.2 O.sub.3) were successfully produced from the thermite mixture of ZrO.sub.2, SiO.sub.2, and aluminum. Walton and Poulos reported some advantages of this method of production as follows: (1) inexpensive precursory powders; (2) low ignition temperature (980.degree. C.); (3) high reaction temperature (+2760.degree. C.); (4) short firing time; and (5) controlling the atmosphere was unnecessary. Therefore, this technique has a significant meaning for industrial production of ceramic, metal and cermet composite materials.
During the American Ceramic Society's 86th Annual Meeting (1984), there were seven presentations about thermite reactions and SHS. One of these presentations disclosed the self-sintering of materials. TiB.sub.2, TiC, and compacts formed from these mixtures have a high potential for weapons systems applications. Such materials produced by conventional processes are expensive because powders with suitable properties for sintering are needed, and high-temperature and high-pressure sintering of these powders are required to produce high-strength materials. On the other hand, by using SHS processing, it may be possible to produce strongly bonded materials with desired phases from precursory powder mixtures by igniting only one end of the compact at room temperature. N. D. Carbin et al. examined the effect of precursory powder characteristics of resulting products in the system Ti-B-C and showed that mixtures containing fine titanium powder are easier to ignite and have slower reaction rates, but the products are more porous than those containing coarse powder. Furthermore, reaction rates in the mixtures using B.sub.4 C for the boron and carbon elements decreased about 100 times and partially sintered products containing TiB.sub.2 and TiC were produced. The resulting products seemed to be considerably porous.
Recently, high-pressure, self-combustion sintering for ceramics, utilizing this SHS processing technique, were demonstrated from cooperative research by Osaka University and Sumitomo Electric Industries Ltd. in Japan. Their attempt is to eliminate the porosity in products produced by SHS by applying high pressure during the SHS process. It is reported that a dense TiB.sub.2 sintered compact was produced in a few seconds by electric ignition of a pressed titanium and boron mixture at 3 GPa; the relative density and microhardness value of the center region in the high pressure reaction cell was 95% and 2000 kg/mm.sup.2 for a 200 g load, respectively. This result suggests that the application of high pressures to SHS processing is very effective in eliminating porosity and has the potential of producing strong, dense ceramic compacts. However, in this high-pressure, self-combustion process, expensive and complicated high-pressure apparatuses and assemblies are required; the expense and complication is similar to conventional high-pressure sintering techniques.
U.S. Pat. No. 4,255,374, to Lemcke et al, discloses a method of compacting interweldable powder materials into a solid body by using a shock wave. This patent does not disclose the use of shock or explosive compression to produce an exothermic chemical reaction or alloying between the powder ingredients. This patent discloses that such chemical reactions or alloying, in particular with respect to diamond powders, would be undesirable because of a general decrease in the hardness and wear resistance of the resulting product.
U.S. Pat. Ser. No. 747,558 discloses an inexpensive method for producing high density compacts of refractory ceramics, ceramic composites, cermets and other high hardness materials. The method comprises applying explosive shock to the compact to produce exothermic sintering and bonding of the compact powder materials.
A comprehensive article on the prior art formation of intermetallic compounds, most of which are formed exothermically, is an article entitled "Intermetallic Compounds: Their Past and Promise," set forth in Metallurgical Transactions A, Volume 8A, Sept. 1977, at page 1327 et seq. This article records the 1976 Campbell Memorial Lecture at the American Society for Metals. Attention is directed particularly to the footnotes and the literature references at the end of this article.
Another article pertaining to prior art exothermic reactions is "Propagation of Gasless Reactions in Solids," by A. P. Hardt and P. V. Phung, 21 Combustion and Flame, pages 77-89 (1973). This article sets forth an analytical study of exothermic intermetallic reaction rates.
Polycrystalline diamond is tougher than single crystalline diamond because of the random orientation of the crystal (no significant cleavage). Both polycrystalline and single crystalline diamond have a high hardness. Thus, natural and synthesized polycrystalline diamond composite materials are useful in cutting tools, wire-drawing dies and rock-drilling bits. Particularly in rock-drilling applications, high hardness and high toughness materials are required. Diamond cutting tools are being used in the automobile, airframe manufacturing and aircraft engine propulsion industries. The work materials in these fields are mainly aluminum alloys with a high silicon content, nickel- and titanium-based alloys, and gray cast irons. In recent years, high speed machine technology has been developed in the industries mentioned above in order to reduce machining costs and to increase productivity. For this purpose, tool materials with a high hardness and high toughness are required. Ceramic tool materials such as Si.sub.3 N.sub.4 -based ceramics, ZrO.sub.2 -based ceramics, and SiC-whisker reinforced alumina are being successfully developed and commercialized for high-speed machining of superalloys and cast irons. However, most attractive and effective tool materials comprise polycrystalline diamond because of the high hardness and high toughness properties. As an example, a comparison of the cutting performance of diamond tool and conventional cemented carbide tool is shown in Table 2. This table was obtained from 208 Science, R. H. Wentorf, R. C. DeVries and F. P. Bundy, p. 873 (1980).
TABLE 2 ______________________________________ Cutting Performance of Sintered Diamond Compared to Cemented Carbide Total Number of Pieces Work Material Cut Per Tool ______________________________________ Silicon-Aluminum SAE 332 Compax Diamond Tool 412000 Cemented Carbide 2400 Rubber filled with nickel and aluminum powder Compax Diamond Tool 6000 Cemented Carbide 140 Type 390 Aluminum Compax Diamond Tool 12000-14000 Cemented Carbide 3000 Glass Filled Polypropylene Compax Diamond Tool 7000 Cemented Carbide 400 ______________________________________
Some kinds of natural polycrystalline diamonds such as framesite, carbonado and ballas, are available for cutting tools and rock-drilling bits, but the amount of these materials is limited. Thus, most of the polycrystalline diamond for industrial applications has to be produced by means of high-pressure sintering techniques using diamond powder. Diamond is a typical, strong, covalently bonded material, and is unstable at high temperatures under ambient pressure. The sintering of diamond powders at high pressures and high temperatures has been studied by many investigators in the prior art. Stromberg and Stevens, 49 Ceramic Bulletin, p. 1030 (1970) and Hall, 169 Science, p. 868 (1970) reported the sintering of diamond with and without additives which were not useful as catalysts for diamond formation. The additives in their compacts served solely as a binder of diamond grains. Sintered diamond compacts with densities of 3.29 to 3.48 g/cm.sup.3 and compressive strengths of 4.4 to 5.8 GPa were produced. However, these references pointed out that the surface transformation of diamond particles into low pressure forms during the sintering was a major problem in using this technique. On the other hand, Katzman and Libby, 172 Science, p. 1132 (1971), and Notsu et al., 12 Materials Research Bulletin, p. 1079 (1977) reported the sintering of diamond powders using additives such as iron, nickel and cobalt, which can act as a catalyst for diamond formation. In this prior art sintering technique, the mechanical properties of the resulting compacts strongly depend upon the amount of additives. It is reported that extensive diamond-diamond bonding was successfully produced in the compacts during the sintering process. Sintered polycrystalline diamond compacts produced by this technique are being commercialized and used for many industrial applications in the prior art.
The use of sintered diamond compacts can be expected to gradually increase in the automobile and aerospace industries in order to machine high-performance superalloys. Such compacts will also be useful in the high-speed machining of conventional and new materials. Especially, in these industries, there will be an increased demand for materials which are capable of cutting high-performance structural ceramics such as Al.sub.2 O.sub.3, Si.sub.3 N.sub.4 and SiC based ceramics and their pure materials. Sintered diamond appears to be the most effective and promising tool material for such machining due to its excellent mechanical and thermal properties. However, in the prior art, the extremely high price of sintered diamond compacts, compared to that of conventional cemented carbide and ceramic tools (about 20 times higher per corner available to cutting), is one of the factors preventing a wide usage of this material in industry. The high price of this material in the prior art is partially due to the high capital and operating costs of high pressure apparatuses in producing polycrystalline diamond compacts.